1. Field of Invention
The present disclosure relates generally to a method of receiving and transmitting satellite signals with an antenna array. More specifically, the present disclosure relates to optimizing performance of an antenna array which comprises several spatially distributed sub-arrays.
2. Description of Prior Art
Long distance communications such as communications between satellites and ground based locations, and even aircraft, typically require high gain antennas. High gain antennas may be implemented as an array of low gain antennas where the signals associated with each low gain antenna are coherently combined to provide performance equivalent to a high gain antenna. In order to coherently combine the signals associated with the low gain antennas these signals must be appropriately aligned in time relative to each other. For narrow band systems an equivalent phase shift may be used to achieve coherent combination. Each low gain antenna may be implemented as a sub-array of radiating elements with commandable relative phase shift applied to the signal associated with each radiating element so that the sub-array beam may be steered electronically. This implementation is attractive in many applications so the low gain antennas will be referred to as sub-arrays. The low gain antennas could also be implemented as parabolic dishes or other known forms of antennas.
To form the optimal composite receive signal from the sub-arrays, time delays need to be applied to the sub-array outputs to correct for the difference in time that the signal reaches each spatially separate sub-array. Otherwise adding the output signals from the respective sub-arrays will not correctly represent the transmitted by the remote station such as a satellite signal. In particular, time delay needs to be applied to the signal collected from a sub-array which is closer to the signal source to compensate for the extra journey time of the signal reaching a sub-array further from the signal source. The magnitude of the required time delay is calculated by multiplying the distance between the two sub-arrays and the cosine of the angle between a line from the further sub-array to the satellite and a line from the further sub-array to the closer sub-array; the product is then divided by the speed of light. Similarly to form an optimal composite transmit signal from an array of sub-arrays it is necessary to add an appropriate time delay to the signal associated with a sub-array which is closer to the signal receiver.
Generally an antenna's sub-arrays are sufficiently close to each other and their relative location with respect to each other is sufficiently well known that the required relative time delays that need to be applied to each sub-array are easily determined based on the direction to the remote communications station (e.g. satellite) and the relative sub-array locations. In some instances however a sub-array may be separated a sufficient distance from another sub-array so that the precision with which the direction to the remote communications station is known is inadequate to satisfactorily combine the signals. Combining the sub-array outputs with inaccurate time synchronization would not yield a representative satellite signal. Another challenge associated with sub-arrays that are spaced further apart is that their relative positions can dynamically change due to vibrations or bending of the support structure. This also limits the effectiveness of signal combining. When integrating an antenna into an air or ground vehicle, there may be severe constraints placed on the relative locations of the sub-arrays based on the vehicle design. So in some cases the sub-arrays are forced to be so far apart that established methods for setting the relative time delay of each sub-array are inadequate for satisfactory long distance communications. Due to the reciprocal nature of electromagnetic transmissions, the performance of a transmit antenna comprising several transmit sub-arrays is degraded in the same manner as a receive antenna comprising several receive sub-arrays. In some narrow band systems it is satisfactory to implement the time delay correction as a phase correction.